The present invention relates to a technology of recording and reproducing information, and especially to, a wobble signal extracting circuit for extracting a wobble signal from an optical disk correctly.
Generally, on the optical disk for recording, a guide groove that indicates a scanning direction of a light beam is formed in a meandering manner. The meandering guide groove is called a wobble. If recording marks are formed on the optical disk based on a recording clock in sync with a meandering frequency of this wobble, an optical disk unit can record information with a predetermined recording density. The relationship between the meandering frequency of the wobble and the recording clock is defined by a standard specification of the optical disk. For example, in DVD-R/RW (Digital Versatile Disk-R/RW), the meandering frequency of the wobble is determined to be 140.65 kHz equivalent to 1/186 times the reference frequency of 26.16 MHz of the recording clock. In DVD+R/RW (Digital Versatile Disk+R/RW), the meandering frequency is 817.5 kHz equivalent to 1/32 times the recording clock.
Moreover, there is a case where modulation indicating position information may be added to the wobble. For example, in DVD+R/RW, the position information is embedded by making a part of the wobble meander in a reverse phase. In CD-R/RW, FM modulation that uses the position information is applied to the wobble frequency itself. Incidentally, in DVD-R/RW, the position information is indicated by pits (land prepits) formed on land adjacent to the guide groove without modulating the wobble.
The optical disk unit determines data recording positions to the optical disk based on the position information being read from the modulated wobble and land prepits and the recording clock in sync with the meandering frequency. In order that a photo detector of the optical disk unit detects the wobble signal indicating the meandering of the wobble, it is equipped with at least two divided light receiving parts along a tangential direction of the guide groove. When the divided light receiving parts detect reflected light rays from the guide groove, the wobble signals are outputted from an inner circumferential side light receiving part and an outer circumferential side light receiving part as mutually reverse phase signals. Therefore, by finding a difference between the signals (a push-pull signal) outputted from the respective light receiving parts on the inside and the outside, the optical disk unit can acquire the wobble signal.
However, an output signal of each light receiving part corresponding to an area where the recording marks were formed is such that a signal of brightness/darkness according to existence/absence of the recording mark (an RF signal) is superposed on the wobble signal. Since this RF signal component is added to the each light receiving part on the inside and the outside in phase, ideally it can be removed by taking a difference between signals outputted from respective light receiving parts on the inside and the outside. However, due to factors of distortion of the optical beam shape focused on the optical disk and a gap between the recording mark and a center of the guide groove, etc., the signal strengths obtained from the respective light receiving parts on the inside and the outside make a difference. Generally, since the strength of the RF signal component is larger than the strength of the signal component accompanying the meandering of the wobble by about one order, even if there is a slight unbalance in output amplitude of the light receiving part, it is often the case where the magnitude of the remaining RF signal component easily becomes a magnitude that cannot be ignored. If the RF signal component remaining in the push-pull signal is large, there is a problem that it becomes difficult to detect the position information being based on the wobble.
As a technology about extraction of the wobble signal, Japanese Unexamined Patent Publication No. 2009-146502 and Japanese Unexamined Patent Publication No. 2005-196846 are disclosed.
Japanese Unexamined Patent Publication No. 2009-146502 discloses a wobble signal extracting circuit that can remove an RF component certainly from the wobble signal with a simple configuration. The wobble signal extracting circuit of Japanese Unexamined Patent Publication No. 2009-146502 has: an adder for acquiring an RF signal component by adding an A+D signal and a B+C signal each of which were detected from the two light receiving parts having been divided into two, respectively; a subtractor for acquiring the wobble signal by subtracting the B+C signal from the A+D signal; a latch comparator for binarizing the RF signal component; a latch comparator for binarizing the wobble signal; and an EXOR circuit for calculating exclusive OR of the binarized RF signal component and the wobble signal. Then, the wobble signal extracting circuit detects an amplitude balance of the A+D signal and the B+C signal based on an operation result of exclusive OR. The wobble signal extracting circuit of Japanese Unexamined Patent Publication No. 2009-146502 makes it possible to make uniform the amplitude balance of the A+D signal and the B+C signal, and makes it possible to extract the wobble signal with high precision.
Japanese Unexamined Patent Publication No. 2005-196846 discloses the wobble signal extracting circuit that removes the RF signal component included in the wobble signal without providing a high-precision band pass filter. This wobble signal extracting circuit has: a first gain amplifier for giving a gain to a first optical disk signal that includes the wobble signal component arising from the wobble formed on the surface of the optical disk and a RF signal component based on recording or reading of data; a second gain amplifier for giving a gain to a second optical disk signal that includes the wobble signal in a reverse phase to the first optical disk signal and the RF signal component; and a subtractor for subtracting an output signal of the second gain amplifier from an output signal of the first gain amplifier. Then, the wobble signal extracting circuit is characterized by evaluating the output signal of the subtractor and controlling at least one of the first and second gain amplifiers based on the result. According to Japanese Unexamined Patent Publication No. 2005-196846, at least one gain of a first gain amplifier and a second gain amplifier is feedback controlled and a high-precision wobble signal that does not include the RF signal component can be extracted.
Moreover, a technology concerning the optical disk unit capable of detecting the wobble signal and the LPP signal with a simple configuration is disclosed in International Patent Publication WO2005/015548. FIG. 1 is a block diagram showing a configuration of the optical disk unit of International Patent Publication No. WO2005/015548. Referring to FIG. 1, the optical disk unit is equipped with: a spindle motor 202; an optical head 203; a servo signal/readout signal generator 204, a focus/track control part 205; a wobble detection balance adjustment circuit 206; a wobble balance detector 207, a wobble detection differential amplifier 208; an LPF (low pass filter) 209; an A/D (analog-to-digital converter) 210; an HPF (high-pass filter) 211; an amplitude detector 212; an LPF 213; a wobble signal detector 214; an A/D 215; a readout signal detector 216; a recording/regenerating PLL circuit 217; a residual component removing circuit 218; an LPF 219; an LPP binarization circuit 220; an LPP address detector 221; a demodulator circuit 222; an error correction code encode/decode circuit 223; a modulation demodulation control circuit 224; a modulator circuit 225; a laser drive waveform generating circuit 226; a laser driver circuit 227; a gating signal generator 228; a CPU 229; a summing amplifier 230; an LPF 231: a binarization circuit 232; a latch circuit 233, and a control signal generating circuit 234.
Principal parts in the optical disk unit of International Patent Publication No. WO2005/015548 will be explained below. The optical head 203 has a tracking detector that detects first and second detection signals based on the reflected light rays of a laser irradiated on the optical disk with first and second detectors placed on the both sides of a division line along the longitudinal direction of the track.
The wobble detection balance adjustment circuit 206 receives the first detection signal and the second detection signal, adjusts them so that the signal level of the first detection signal and the signal level of the second detection signal may become equal to each other, and outputs respective detection signals. A wobble detection differential amplifier circuit 208 generates the wobble detection differential signal that is a difference between the first detection signal and the second differential signal whose signal levels were adjusted by the wobble detection balance adjustment circuit 206. The A/D 210 converts the wobble detection differential signal generated by the wobble detection differential amplifier circuit 208 into a digital signal. The wobble signal detector 214 detects the wobble signal based on the wobble detection differential signal that was converted into the digital signal by the A/D 210.
The summing amplifier 230 generates a sum signal that is a sum of the first detection signal and the second detection signal whose signal levels were adjusted by the wobble detection balance adjustment circuit 206. The binarization circuit 232 converts the sum signal generated by the summing amplifier 230 into a binarized signal by comparing it with a signal of a predetermined level. The latch circuit 233 converts the output signal of the binarization circuit 232 into a recording timing signal by latching it with a conversion clock of the A/D 210 or a clock having a frequency that is an integer times the conversion clock. The control signal generating circuit 234 generates a control signal for removing the residual signal component that is a residual component of the recording signal included in the wobble detection differential signal based on the recording timing signal that is an output signal of the latch circuit 233 and the wobble detection differential signal that was converted into the digital signal.
The residual component removing circuit 218 extracts a land prepit detection signal by removing the residual signal component included in the wobble detection differential signal that was converted into a digital signal based on the control signal supplied from the control signal generating circuit 234. An LPP address detector circuit 221 detects address information based on the land prepit detection signal outputted from the residual component removing circuit 218. Since the optical disk unit of International Patent Publication No. WO2005/015548 can acquire respective optimum conditions for detection of the wobble signal and the LPP signal using one wobble detection balance adjustment circuit 206 and one wobble detection differential amplifier 208, it becomes possible to detect the wobble signal and the LPP signal with a simple configuration.